1. Field of the Disclosure
The present invention relates to a ventilation inlet; in particular, but not exclusively, to a ventilation inlet for a nacelle cavity of a gas turbine engine, for example a fire zone ventilation inlet.
2. Description of the Related Art
A gas turbine engine 10, to which both the present invention and the prior art ventilation inlets are applicable, is shown in FIG. 1. The engine 10 comprises an air intake 12 and a propulsive fan 14 that generates two airflows A and B. The gas turbine engine 10 comprises, in axial flow A, an intermediate pressure compressor 16, a high pressure compressor 18, a combustor 20, a high pressure turbine 22, an intermediate pressure turbine 24, a low pressure turbine 26 and an exhaust nozzle 28.
A nacelle 30 surrounds the gas turbine engine 10 and defines, in axial flow B, a bypass duct 32. The radially inner extent of the bypass duct 32 is defined by an annular inner wall 34.
The annular inner wall 34 typically defines at least two exemplary fire zones, zone z2 and zone z3, (also referred to as a nacelle cavity) that are axially separated by a barrier wall 36. Downstream of the barrier wall 36 there is typically provided an array of ventilation inlets that are preferably equi-angularly spaced around at least a portion of the circumference of the annular inner wall 34 to permit air to flow from the bypass duct 32 into fire zone z3 to ventilate and purge it.
A prior art ventilation inlet 38 typically takes the form of a static pressure tapping, being a pipe directed radially so that it extends substantially perpendicularly to the flow through the bypass duct 32, as shown in FIG. 2.
The flow through such a prior art ventilation inlet is driven via the static-to-static pressure ratio between the bypass duct (in the region of the respective inlet opening) and the nacelle cavity. This pressure differential can be of the order of 1-1.6:1, dependent upon engine condition.
The flow rate through the ventilation inlets is a regulatory requirement to provide an ‘absence of accumulation of flammable fluid vapours within the nacelle’ so as to minimise the risk of fire. Furthermore, the ventilation additionally acts as a transport medium for fire extinguishant around the nacelle cavity.
The final function of this flow is to provide thermal management to engine units, accessories and the nacelle cavity boundary itself.
As can be seen from FIG. 2, the prior art ventilation inlets are of a fixed, or static, configuration and are fabricated from materials that are inherently fireproof so as to prevent increase of flow through the inlet and consequently into the nacelle cavity during a fire event, thereby exacerbating the fire. These materials are usually steel or titanium. The fixed, or static, nature of these inlets also provides a capability that is consequently not subject to reliability assessment—i.e. they are fixed geometries and nothing can malfunction.
However, a key disadvantage of the prior art ventilation inlets is that flow into the nacelle cavity, and hence the size and configuration of the ventilation inlet, must be selected to provide sufficient purging of the nacelle cavity at low power engine conditions so as to satisfy the regulatory requirements. However, selecting the size and configuration of the ventilation inlet on this basis results in an increased flow through the inlet, and thus significant over purging of the nacelle cavity, during certain flight conditions e.g. cruise. This flow excess results in a disadvantageous effect on specific fuel consumption and the propulsive efficiency of the engine 10.